Sporadic Scope Issues With Injected Fakes In Rust's Injectorpp

This article delves into a peculiar issue encountered while using injectorpp for Rust, where injected fakes are sporadically called outside the intended scope of their definition. This behavior can lead to unexpected test failures and a lack of confidence in the isolation of test cases.

The Problem: Scope Confusion with Injected Fakes

The core issue revolves around the unexpected behavior of injected fakes within the injectorpp framework. Specifically, an injected fake, designed to mock a function's behavior within a particular test scope, is sometimes invoked in a different test scope. This violates the principle of test isolation, where each test should operate independently without influencing others. When injected fakes bleed over into unintended scopes, test results become unreliable and debugging becomes significantly more challenging.

This article will dissect this problem, examine a concrete test scenario that demonstrates the issue, and discuss potential causes and solutions. The goal is to provide a comprehensive understanding of this behavior and offer strategies for mitigating its impact on Rust projects employing injectorpp.

Test Scenario: Demonstrating the Scope Issue

To illustrate the issue, consider the following simplified Rust project structure:

.
├── main.rs
└── test_module
    ├── first.rs
    ├── mod.rs
    └── second.rs

This structure includes a test_module directory containing three files: first.rs, second.rs, and mod.rs. The key files for demonstrating the issue are first.rs and second.rs, which define two simple structs, First and Second, respectively.

first.rs

pub struct First {
    a: i32,
}

impl First {
    pub fn new() -> Self {
        First { a: 0 }
    }

    pub fn get(&self) -> i32 {
        self.a
    }
}

#[cfg(test)]
mod tests {
    use injectorpp::interface::injector::*;

    use crate::test_module::first::First;

    #[test]
    fn get_call_mock() {
        let mut injector = InjectorPP::new();
        injector
            .when_called(injectorpp::func!(fn (First::get)(&First) -> i32))
            .will_execute(injectorpp::fake!(func_type: fn(_i: &First)-> i32, returns: 7));

        assert_eq!(First::new().get(), 7);
    }
}

The first.rs file defines a struct First with a method get that returns an integer. The test module within first.rs uses injectorpp to inject a fake implementation for the get method, causing it to return 7 instead of the default value 0. This test case, get_call_mock, verifies that the injected fake works as expected within its scope.

second.rs

use super::first::First;

pub struct Second {
    tm: First,
}

impl Second {
    pub fn new() -> Self {
        Second { tm: First::new() }
    }

    pub fn get(&self) -> i32 {
        self.tm.get()
    }
}

#[cfg(test)]
mod tests {
    use super::Second;

    #[test]
    fn get_do_not_call_mock() {
        assert_eq!(Second::new().get(), 0);
    }
}

The second.rs file defines a struct Second that contains an instance of First. The get method of Second simply calls the get method of its First instance. The test case get_do_not_call_mock in the test module of second.rs asserts that calling Second::new().get() should return 0, as the fake injected in first.rs should not affect this test scope.

Expected Behavior: Isolated Test Scopes

The expected behavior is that the injected fake within the get_call_mock test in first.rs should only affect calls to First::get within that specific test function. The get_do_not_call_mock test in second.rs should always return 0 because it creates a new instance of Second, which in turn creates a new instance of First, and the fake should not persist across different test scopes.

Actual Behavior: Sporadic Test Failures

However, running the tests repeatedly using a loop like the following reveals the issue:

for i in $(seq 0 300); do cargo test; done

Sporadically, the get_do_not_call_mock test in second.rs fails with an assertion error, indicating that it returned 7 instead of the expected 0. This demonstrates that the injected fake from first.rs is somehow being applied in the scope of the second.rs test, violating test isolation.

running 2 tests
test test_module::first::tests::get_call_mock ... ok
test test_module::second::tests::get_do_not_call_mock ... FAILED

failures:

---- test_module::second::tests::get_do_not_call_mock stdout ----
thread 'test_module::second::tests::get_do_not_call_mock' panicked at src/test_module/second.rs:23:9:
assertion `left == right` failed
  left: 7
 right: 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_module::second::tests::get_do_not_call_mock

test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

This output clearly shows that the get_do_not_call_mock test, which should have returned 0, instead returned 7, indicating that the injected fake from the first.rs test scope was inadvertently applied.

Analyzing the Cause of the Issue

The sporadic nature of the test failures suggests a potential issue with how injectorpp manages the scope of injected fakes. While the library is intended to provide test isolation, there might be underlying mechanisms that, under certain conditions, lead to the leakage of fakes across test boundaries.

Possible causes for this behavior include:

1. Shared Global State: If injectorpp uses a shared global state to store injected fakes, there might be race conditions or incorrect cleanup procedures that cause fakes to persist beyond their intended scope. This is a common pitfall in mocking frameworks that are not designed to be fully thread-safe or properly scoped.
2. Incorrect Scope Management: The scoping mechanism within injectorpp might have a flaw that, under specific circumstances, fails to correctly isolate fakes. This could involve issues with how injectors are created, used, and disposed of within different test modules.
3. Test Execution Order: The order in which tests are executed can sometimes influence the outcome, especially if there are shared resources or global state involved. If the test in first.rs runs before the test in second.rs, and the fake is not properly cleared, it might inadvertently affect the subsequent test.
4. Concurrency Issues: Rust's testing framework can run tests concurrently. If injectorpp is not thread-safe, concurrent access to the injector's state could lead to unpredictable behavior and the leakage of fakes.

Strategies for Mitigating the Issue

Addressing this issue requires a multi-faceted approach, involving both temporary workarounds and potential fixes within the injectorpp library itself. Here are some strategies to consider:

1. Isolate Injector Instances: Ensure that each test or test module has its own independent instance of the InjectorPP. Avoid sharing injectors across different test scopes. This helps to minimize the risk of fakes bleeding over from one test to another.
2. Explicitly Clear Fakes: If injectorpp provides a mechanism to explicitly clear or reset the injector's state, use it after each test case that injects fakes. This helps to ensure that fakes are removed before the next test begins.
3. Reduce Global State: If the issue stems from shared global state within injectorpp, consider refactoring the code to minimize the use of global variables or static data structures. Favor dependency injection and local state management.
4. Thread Safety: If concurrency is a concern, ensure that injectorpp is used in a thread-safe manner. This might involve using mutexes or other synchronization primitives to protect shared resources. If the library itself is not thread-safe, consider using a thread-local injector instance for each test thread.
5. Report the Issue: The most important step is to report the issue to the maintainers of the injectorpp library. Provide a clear and concise bug report with a reproducible test case, such as the one described in this article. This will help the maintainers to identify the root cause of the problem and develop a fix.
6. Consider Alternative Mocking Frameworks: If the issue persists and significantly impacts testing reliability, it might be necessary to consider alternative mocking frameworks for Rust. There are several other options available that might provide better test isolation and stability.

Conclusion: Ensuring Test Isolation with Mocking

The issue of injected fakes being called outside of their intended scope highlights the importance of test isolation in software development. Mocking frameworks like injectorpp are powerful tools for isolating units of code during testing, but they must be used carefully to avoid unintended side effects. By understanding the potential causes of scope leakage and implementing appropriate mitigation strategies, developers can ensure that their tests are reliable and provide accurate feedback on the behavior of their code.

This article has presented a concrete example of this issue, analyzed potential causes, and suggested strategies for mitigating the problem. By addressing these concerns, Rust developers can leverage the benefits of mocking without compromising the integrity of their test suites. The key takeaway is that vigilance and a thorough understanding of the mocking framework are essential for maintaining reliable and trustworthy tests.